In the past, the conventional dual slope analog to digital (A/D) converter processed analog signals by establishing an accurate zero signal baseline (auto zero) for the converter prior to integrating the analog signal. After integrating the analog signal, an opposite polarity reference signal was integrated back to the zero baseline. The time required to reach the zero baseline was measured in digital clock signals, or periods, which established the digital representation of the analog signal. Because of the unavoidable integrator overshoot pass the zero baseline, to have any degree of conversion accuracy it has always been necessary to re-establish the accurate zero baseline (auto zero) of the converter prior to taking additional readings.
With the above system, for a given conversion accuracy, the rate at which conversions are made can only be speeded up by increasing the frequency of the clock. In certain applications, both high rate low accuracy samples, and low rate higher accuracy conversions are required, but the penalties associated with a high frequency clock are not acceptable.
Previously, there have been two approaches to making simultaneous analog to digital conversions at two different rates with different degrees of accuracy. The first required separate conversion systems for the two rates which resulted in higher costs for the two separate systems. The second was by providing a fast clock period for converting at the fastest rate at the full desired resolution but with poor noise rejection characteristics. Such conversions would be followed by digital filtering for lower noise, and therefore higher accuracy, at a slower rate.
However, a fast clock period requires increased attention to noise and bandwidth effects, and requires critical conversion system components, leading towards increases both in power consumption and in costs.
Heretofore, there has been no way of applying dual slope A/D techniques and economically obtaining both a high rate of analog to digital sampling with lowered accuracy while obtaining a lower rate of conversion with higher accuracy. With a dual rate conversion system, it would be possible to display an analog signal represented, by a high speed bar graph, and a slower numeric-digital display.